1. Field of the Invention
This invention relates generally to the field of frequency synthesizer systems and more particularly to such systems for use in communication transceivers. It further relates to a battery saving system for synthesized transceivers.
2. Description of the Prior Art
It is recognized that for many years there has been an ever increasing demand for FM two-way portable radios. This stems from the increasing use of portable radio communications in business, industry and government. The level of sophistication of the communication networks utilized by various institutions has constantly risen, and with that increase has come the demand for the capability of the portable transceivers to operate over a much larger number of frequencies. The majority of two-way FM portable radios currently in use are those which employ conventional crystal controlled frequency channel elements. Thus, as each portable radio is required to either transmit or receive on more than one frequency, an additional crystal controlled channel element must be included and electrically actuated so that both transmission and/or reception can occur.
As is the case with the several networks or systems, many different frequency combinations are grouped into communication channels from the available frequencies within a given area. Thus, any portable transceiver customer must specify to the manufacturer the desired pairs of receive and transmit frequencies for the several communication channels which are required in portable transceivers for a selected area. With such an order, the corresponding crystals to enable two-way communication on those channels must be manufactured and inserted into the transceiver. Frequently, design modifications must be made to accommodate the additional channel elements resulting in an increased size and weight of unit and a greatly increased cost.
The required long lead time for the manufacture of multifrequency transceivers to meet the market demand has caused a general trend toward the use of frequency synthesis in the design of FM two-way portable radios. The combination of a master oscillator signal with a secondary signal in a suitable mixer can provide the choice of a number of controlled frequencies. If a stable variable frequency oscillator is substituted for a fixed crystal oscillator and a digital frequency synthesis technique is employed, a virtually unlimited number of discrete frequencies directly related to the frequency of the master oscillator are available. Instead of providing a plurality of individual channel elements suitable for each individual user's purpose, a manufacturer can provide one or more crystal controlled oscillators and a programmable memory which can be modified at the factory to conform to an individual user's required frequencies. This enables a manufacturer to assemble virtually all of his transceiver units in the same way and near the last step in the manufacturing process insert the memory programmed to the individual user's frequency requirements.
One of the basic problems with the use of frequency synthesized portable radios is the limited power available for the portable hand-held units. Thus, any frequency synthesis system must not be wasteful of the limited battery capacity available in the portable units. Portable radios are frequently designed to operate efficiently for an eight-hour duration on some predetermined duty cycle designating the percentage of time it would be used to receive, transmit and to remain on standby. As, for example, a 5-5-90 would represent 5% receive, 5% transmit and 90% standby, and this forms a standard by which the power consumption of various radios may be compared. What may also be seen is that such a radio spends the vast majority of its time in a standby mode.
One prior art receiver has a switch connected between the battery and the frequency synthesizer whose output is coupled directly to the voltage controlled oscillator and to a capacitor operating as a voltage memory. When the battery switch is periodically opened, the power to the frequency regulation is disrupted, its output is disconnected from the VCO and the voltage memory capacitor is used to supply voltage to the VCO and hence maintain the tuning. The latest state of the art technology utilizes CMOS for the dividers thus reducing their overall current drain. It also fails to recognize that the highest current drawing component in a synthesized transceiver which is designed to match the performance characteristics of crystal controlled transceivers is the VCO. Therefore, the switched regulating circuit of the prior art can best only provide a small enhancement of battery life for a synthesized transceiver. It also makes no provision to guarantee that the synthesizer will not go out of a lock condition when the battery saving operation is imposed.
In addition, it has been found that various design implementations of digital frequency synthesizers do not meet the rigid specifications which are applied to radios which employ crystal controlled channel elements. It is well recognized in the art that crystal controlled channel elements produce extremely well defined frequencies. Present designs of digital frequency synthesizers can provide the same degree of accurate tuning in portable transceivers as can the crystal controlled channel elements but at the cost of higher current drain primarily due to the VCO.
The problem is therefore to find a digital frequency synthesized transceiver system for portable transceivers programmability which will provide the size reduction, cost efficiency, and most importantly, the power conservation that a multitude of users require yet can stay within the rigid performance specifications that apply to the use of crystal controlled elements.